Over temperature and under temperature detection are important for the protection of personnel, equipment, and agricultural crops. Electronic equipment will fail if allowed to exceed specified maximum and minimum temperatures. Incipient mechanical equipment failure can be detected by sensing overheating and shutting down the equipment. Ammunition dumps, power cables, coal conveyors, etc. must all be operated below maximum temperature levels to avoid damage. Agricultural crops must be kept from freezing to survive. Certain chemical processes and fluid transport pipes must be kept at temperatures well above ambient to avoid adverse chemical reactions or increased viscosity. These are but a few examples of extended or spatially distributed systems where knowledge of undesirable temperature excursions is essential.
Heating and cooling systems are often provided to avoid undersirable temperature excursions. Such systems, however, are generally energy intensive, and, with the advent of the energy crisis, expensive to operate. Furthermore, heating and cooling systems are often operated when they are not needed due to the lack of over- or under-temperature information. Finally, such systems often fail without warning and without any indication to the user, resulting in undesirable temperature excursions which often lead to catastrophic failures and safety hazards.
Temperature excursions are conventionally measured by a number of devices which change a physical property with temperature. Perhaps the simplest device is the thermostatic switch or thermostat which either opens or closes an electrical circuit when the temperature rises above or falls below a predetermined value. The thermostat is a discrete device but has been used to monitor extended or spatially distributed systems for temperature excursions through the use of multiplexing or by running individual wires to each of the thermostats. Multiplexing requires an electronic circuit and electrical power at the location of each thermostat to encode the information for transmission. The use of individual wires produces a heavy, inflexible, expensive, and generally impractical cable when large numbers of thermostats are required.
The instant invention improves over the above described techniques by providing a light, flexible cable containing only three conductors and thermostatic devices distributed along the length of the cable. Multiplexing circuits and individual wires are no longer necessary to detect and locate temperature excursions.
Commonly assigned U.S. patent application Ser. No. 229,913, now U.S. Pat. No. 4,372,693 discloses a temperature excursion and sensing device. The instant invention is an improvement to U.S. patent application Ser. No. 229,913, now U.S. Pat. No. 4,372,693 which is incorporated herein by reference wherein the improvement comprises the inclusion of a third conductor permitting a positive alarm/no alarm detection method independent of capacitance measurements, bi-directional sensing and branched configurations of sensing cable. Specifically, the third conductor permits the sensing of more than one temperature excursion by measuring from both ends of the cable and also permits branched circuits.